Firefox thinks I'm on a captive portal but I'm at home. "You must log in to this network before you can access the Internet"

Hi Maybe this will help https://support.mozilla.org/en-US/questions/1313356

Out of curiosity, if you try to open: http://detectportal.firefox.com/canonical.html does it open without any errors or weird things happening? (It normally redirects through several hoops to a support page here.) Something on your network or ISP might be messing with correctly rendering that URL, which might make Firefox think you're behind some network protections unable to see "outside".